Marshall 8100 head hum query

[Roob65]

Hi Folks,
I'm new to the forum and amp electronics in general but I'm using the current lockdown to learn some new skills.
I aquired a couple of Marshall 8100 heads recently which were sold as 'needing attention'. I spent some time fixing obvious cold joints and replacing a couple of bulging caps.
I've got one of them up and running and have to say I really enjoy it but it does hum rather loudly (sounds to me like 50hz mains - UK).

Anyway I thought I'd try and sort it out rather than just live with it and have spent the last couple of days reading every post on every forum I could find which has lead me here.
At this stage I'm still teaching myself and working my way through the schematic so don't need a ton of suggestions, I just wanted to check my understanding on a point.

The hum is not impacted by any controls and does not increase when gain/master etc. is increased. I was originally working on the conclusion that as the hum does not increase when the volume is increased (VR13 in the schematic - it won't let me attach it possibly becase I have a new account?) then it must be a problem at the power amp stage but I have now found the hum is present on the 'effects send' line which means it must be before VR13?

Just wanted to check my understanding was correct or if there is something I'm missing.
Thanks for taking the time to read. Hope everyone is safe and well.
cheers,
Roob

[Chuck H]

Is this your schematic?

https://schematicheaven.net/marshall..._8100_8412.pdf

[Roob65]

Yes that's the one thanks for attaching it!

[Chuck H]

Do both amps have the same problem?

[Roob65]

I've not spent any time on the 2nd one and haven't hooked it up as it's missing a couple of pots which I'm waiting on getting delivered.

[Jon Snell]

You say that none of the controls make any difference to the 50HZ amplitude.
Disconnect the wiper of the master vol control. Does it go quiet, if it does your hum is coming from the pre amp section and you can disregard the main amplifier and power section.
Otherwise;
Connect a temporary wire to the junction of C40 & C41, (that is the main amplifier ground).
Temporarily connect the wire to the junction of C18 & R25.
If the hum stops it may be the volume control ground is not good connection, try it on the master vol slider then the ground point and work backwards towards the ECC83.
That will at least separate the power amp from the power supply ground and the pre amp.

[Roob65]

Thanks for the suggestion Jon I'll give that a try and get back to you. Out of interest, and because I'm learning, does that mean my theory about it's presence on the effects send is flawed and I need to go back to the books haha!? Thanks again for your reply.

[Roob65]

Quick update as I can't do anything else today as other stuff on my plate. I removed VR13 and the hum is still there. I attached a temp wire between C40 & C41 and C18 & R25 (with VR13 still removed) and no change. Not tried anything else yet.

[Chuck H]

Quick update as I can't do anything else today as other stuff on my plate. I removed VR13 and the hum is still there. I attached a temp wire between C40 & C41 and C18 & R25 (with VR13 still removed) and no change. Not tried anything else yet.

Can't say yet regarding the effects loop. You may be seeing hum in both locations because you have two problem or still possibly the same problem. As yet to be determined. But either way you should check for DC on the speaker when the hum is happening. If you do (possibly a bad output transistor) then you shouldn't use the speaker anymore before the problem is fixed.

[Roob65]

Thanks Chuck. I had previously checked for DC across the speaker terminals but decided to check again this morning. Interestingly I initially get a low negative value (around -16mV) but it slowly started to creep so left it a couple of minutes and it stabilised at around +50mV. Seems a bit high? Homefully the pots for the other amp will get delivered today so I can do a comparison (if it works at all!).

[Enzo]

A few millivolts is not an issue. 50mv is 0.05 volts, which is not an issue.

AMps have three parts: preamp, power amp, and power supply. A hummy power supply would affect both other parts. You have I don't know what, maybe 35v on the power amp rails. There will be a small amount of ripple. But your preamp uses 15v rails. Check them both. Are they up to voltage and CLEAN? I don't care if they are exactly 15v. 14.5 or 15.2 or something is close enough. What I do care about is are they both about the same as each other?

There is a lot of preamp after the preamp. Power amp starts after VR13.

[Roob65]

Thanks for that Enzo. I have done so much reading over the last couple of days I thought I had seen somewhere anything over 20mV was suspect. So tempted to try and pick up a second hand scope - who can resist a new tool right? Haha! Anyway, I'll get to checking and report back after work.

[Roob65]

ok managed to grab a minute during lunch. DC measured at ZD1/ZD2 +14.7v and -14.9v
How clean is a different matter...I'll have a read up and see if my cheapo meter is up to the challenge.

[nosaj]

ok managed to grab a minute during lunch. DC measured at ZD1/ZD2 +14.7v and -14.9v
How clean is a different matter...I'll have a read up and see if my cheapo meter is up to the challenge.

Use your scope an you can see what it looks like
nosaj

[Roob65]

no scope, just been thinking of picking up a used one (second-hand). My DMM is junk so just considering getting a new one or going down the scope route.